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<span id="Fortran_002dinterface-routines"></span><div class="header">
<p>
Next: <a href="FFTW-Constants-in-Fortran.html" accesskey="n" rel="next">FFTW Constants in Fortran</a>, Previous: <a href="Calling-FFTW-from-Legacy-Fortran.html" accesskey="p" rel="prev">Calling FFTW from Legacy Fortran</a>, Up: <a href="Calling-FFTW-from-Legacy-Fortran.html" accesskey="u" rel="up">Calling FFTW from Legacy Fortran</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<span id="Fortran_002dinterface-routines-1"></span><h3 class="section">8.1 Fortran-interface routines</h3>

<p>Nearly all of the FFTW functions have Fortran-callable equivalents.
The name of the legacy Fortran routine is the same as that of the
corresponding C routine, but with the &lsquo;<samp>fftw_</samp>&rsquo; prefix replaced by
&lsquo;<samp>dfftw_</samp>&rsquo;.<a id="DOCF9" href="#FOOT9"><sup>9</sup></a>  The single and long-double precision
versions use &lsquo;<samp>sfftw_</samp>&rsquo; and &lsquo;<samp>lfftw_</samp>&rsquo;, respectively, instead of
&lsquo;<samp>fftwf_</samp>&rsquo; and &lsquo;<samp>fftwl_</samp>&rsquo;; quadruple precision (<code>real*16</code>)
is available on some systems as &lsquo;<samp>fftwq_</samp>&rsquo; (see <a href="Precision.html">Precision</a>).
(Note that <code>long double</code> on x86 hardware is usually at most
80-bit extended precision, <em>not</em> quadruple precision.)
</p>
<p>For the most part, all of the arguments to the functions are the same,
with the following exceptions:
</p>
<ul>
<li> <code>plan</code> variables (what would be of type <code>fftw_plan</code> in C),
must be declared as a type that is at least as big as a pointer
(address) on your machine.  We recommend using <code>integer*8</code> everywhere,
since this should always be big enough.
<span id="index-portability-6"></span>

</li><li> Any function that returns a value (e.g. <code>fftw_plan_dft</code>) is
converted into a <em>subroutine</em>.  The return value is converted into
an additional <em>first</em> parameter of this subroutine.<a id="DOCF10" href="#FOOT10"><sup>10</sup></a>

</li><li> <span id="index-column_002dmajor-2"></span>
The Fortran routines expect multi-dimensional arrays to be in
<em>column-major</em> order, which is the ordinary format of Fortran
arrays (see <a href="Multi_002ddimensional-Array-Format.html">Multi-dimensional Array Format</a>).  They do this
transparently and costlessly simply by reversing the order of the
dimensions passed to FFTW, but this has one important consequence for
multi-dimensional real-complex transforms, discussed below.

</li><li> Wisdom import and export is somewhat more tricky because one cannot
easily pass files or strings between C and Fortran; see <a href="Wisdom-of-Fortran_003f.html">Wisdom of Fortran?</a>.

</li><li> Legacy Fortran cannot use the <code>fftw_malloc</code> dynamic-allocation routine.
If you want to exploit the SIMD FFTW (see <a href="SIMD-alignment-and-fftw_005fmalloc.html">SIMD alignment and fftw_malloc</a>), you&rsquo;ll
need to figure out some other way to ensure that your arrays are at
least 16-byte aligned.

</li><li> <span id="index-fftw_005fiodim-2"></span>
<span id="index-guru-interface-4"></span>
Since Fortran 77 does not have data structures, the <code>fftw_iodim</code>
structure from the guru interface (see <a href="Guru-vector-and-transform-sizes.html">Guru vector and transform sizes</a>) must be split into separate arguments.  In particular, any
<code>fftw_iodim</code> array arguments in the C guru interface become three
integer array arguments (<code>n</code>, <code>is</code>, and <code>os</code>) in the
Fortran guru interface, all of whose lengths should be equal to the
corresponding <code>rank</code> argument.

</li><li> The guru planner interface in Fortran does <em>not</em> do any automatic
translation between column-major and row-major; you are responsible
for setting the strides etcetera to correspond to your Fortran arrays.
However, as a slight bug that we are preserving for backwards
compatibility, the &lsquo;<samp>plan_guru_r2r</samp>&rsquo; in Fortran <em>does</em> reverse the
order of its <code>kind</code> array parameter, so the <code>kind</code> array
of that routine should be in the reverse of the order of the iodim
arrays (see above).

</li></ul>

<p>In general, you should take care to use Fortran data types that
correspond to (i.e. are the same size as) the C types used by FFTW.
In practice, this correspondence is usually straightforward
(i.e. <code>integer</code> corresponds to <code>int</code>, <code>real</code>
corresponds to <code>float</code>, etcetera).  The native Fortran
double/single-precision complex type should be compatible with
<code>fftw_complex</code>/<code>fftwf_complex</code>.  Such simple correspondences
are assumed in the examples below.
<span id="index-portability-7"></span>
</p>
<div class="footnote">
<hr>
<h4 class="footnotes-heading">Footnotes</h4>

<h5><a id="FOOT9" href="#DOCF9">(9)</a></h3>
<p>Technically, Fortran 77 identifiers are not
allowed to have more than 6 characters, nor may they contain
underscores.  Any compiler that enforces this limitation doesn&rsquo;t
deserve to link to FFTW.</p>
<h5><a id="FOOT10" href="#DOCF10">(10)</a></h3>
<p>The
reason for this is that some Fortran implementations seem to have
trouble with C function return values, and vice versa.</p>
</div>
<hr>
<div class="header">
<p>
Next: <a href="FFTW-Constants-in-Fortran.html" accesskey="n" rel="next">FFTW Constants in Fortran</a>, Previous: <a href="Calling-FFTW-from-Legacy-Fortran.html" accesskey="p" rel="prev">Calling FFTW from Legacy Fortran</a>, Up: <a href="Calling-FFTW-from-Legacy-Fortran.html" accesskey="u" rel="up">Calling FFTW from Legacy Fortran</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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